Since a superconducting phenomenon was discovered with respect to mercury in 1911, superconductivity has been found out with respect to many compounds and has been practiced in the form of, e.g., superconducting magnets and magnetic sensors (SQUID). Further, after discovery of a high-temperature superconductor (i.e., perovskite-type copper oxide), research and development of materials have been intensively performed aiming at room-temperature superconductors, and superconducting compounds having superconducting transition (critical) temperature (Tc) of 100K or higher have been found out.
Understanding of the superconductivity developing mechanism in the perovskite-type copper oxide has also been progressed (e.g., Non-Patent Documents 1 and 2). Further, as compounds containing transition metal ions other than copper or as novel compounds, Sr2RuO4 (Tc=0.93K) (Non-Patent Document 3), magnesium diboride (Tc=39K) (Non-Patent Document 4 and Patent Document 1), Na0.3CoO2.1.3H2O (Tc=5K) (Non-Patent Document 5 and Patent Documents 2 and 3), etc. have been newly found out.
It is known that a strongly-correlated electron system compound exhibiting a greater interaction between conduction electrons in comparison with a conduction band width becomes a superconductor having a higher Tc at a higher possibility when the number of d-electrons has a specific value. The strongly-correlated electron system is realized in layered compounds having transition metal ions in skeleton structures. In many of those layered compounds, electrical conductivity is similar to that of a Mott insulator and an antiferromagnetic interaction acts between electron spins so as to array the electron spins in an antiparallel relation.
However, regarding La2CuO4 as one perovskite-type copper oxide, for example, it is confirmed that, in La2-xSrxCuO4 which is obtained by adding Sr2+ to the La3+ site and by substituting part of La with Sr, the compound comes into an itinerant electron state exhibiting metallic conductivity when a value of x is in the range of 0.05 to 0.28, a superconductor state is observed at low temperature, and maximum Tc=40K is obtained at x=0.15 (Non-Patent Document 6).
Recently, the inventors have found out that new strong electron correlation compounds containing Fe as a main component, i.e., LaOFeP and LaOFeAs, are superconductors, and have filed a patent application (Patent Document 4 and Non-Patent Document 7). More specifically, in a strong electron correlation system, an itinerant electron state exhibiting metallic conductivity is generated when the number of d-electrons has a specific value, and there occurs transition to a superconducting state at a particular temperature (superconducting transition temperature) or below when the temperature is lowered. Further, Tc of such a superconductor is changed over the range of 5K to 40K depending on the number of conduction carriers. Moreover, in conventional superconductors such as Hg and Ge3Nb, electron pairs (Cooper pairs) attributable to heat fluctuation (lattice vibration) of crystal lattices are regarded as developing a superconductivity generating mechanism (BCS mechanism). On the other hand, regarding superconductivity of the strong electron correlation system, electron pairs attributable to heat fluctuation of electron spins are regarded as developing a superconductivity generating mechanism. Since then, it has been found out that LaONiP is also a superconductor (Non-Patent Documents 8 to 10).
In the above-mentioned superconducting compounds, electron pairs are in a spin singlet state where respective electron spins are arrayed antiparallel to each other. In Sr2RuO4 (Tc=0.93K) (Non-Patent Document 3), etc., however, superconductivity attributable to spin triplet electron pairs in which electron spins of the electron pairs are arrayed parallel to each other has been lately found out. Such a phenomenon is presumably based on the fact that those spin pairs exhibit a ferromagnetic interaction between their electron spins (i.e., an interaction to align the spins parallel to each other). For that type of superconductor, it is considered that a critical magnetic field at which the superconducting state is broken by a magnetic field is strong. Accordingly, that type of superconductor is superior when it is used in a ferromagnetic field (for example, when it is used as an inner coil in the case of generating a magnetic field in tandem).    Non-Patent Document 1: Nobuo Tsuda, Keiichiro Nasu, Atushi Fujimori, and Kiichi Shiratori, revised “Denki Dendosei Sankabutsu (Electrically Conductive Oxides)”, pp. 350-452, Shokabo Publishing Co., Ltd., (1993)    Non-Patent Document 2: Sadamichi Maekawa, Oyo Butsuri (Applied Physics), Vol. 75, No. 1, pp. 17-25, (2006)    Non-Patent Document 3: Y. Maeno, H. Hashimoto, K. Yoshida, S. Nishizaki, T, Fujita, J. G. Bednorz, F. Lichtenberg, Nature, 372, pp. 532-534 (1994)    Non-Patent Document 4: J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, and J. Akimitsu, Nature, 410, pp. 63-64, (2001)    Non-Patent Document 5: K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R. A. Dilanian, T. Sasaki, Nature, 422, pp. 53-55, (2003)    Non-Patent Document 6: J. B. Torrance et al., Phys. Rev., B40, pp. 8872-8877, (1989)    Non-Patent Document 7: Y. Kamihara et al., J. AM. CHEM. SOC., (Published on Web Jul. 15, 2006), 128, 10012-10013 (2006)    Non-Patent Document 8: T. Watanabe et al., Inorganic Chemistry, 46(19) (2007) 7719-7721 (Published on Web 17 Aug. 2007)    Non-Patent Document 9: T. Watanabe et al., Extended Abstracts (The 68th Autumn Meeting, 2007); The Japan Society of Applied Physics, No. 1, P. 275, 4γ-ZE-2 (2007)    Non-Patent Document 10: M. tegel et al., Solid State Science, 10 (2008)193-197 (Published on Web 2 Sep. 2007)    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-211916    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-262675    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-350331    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2007-320829